Fibers, such as glass fibers, are commonly used as reinforcements for synthetic polymer composites. These fiber reinforced composites are desirable for their combination of light weight and strength and are useful in a variety of applications including automobile components and housings for computers.
Conventional techniques may be used to make fiber reinforced composites and usually involve placing a pre-form in a mold and forming a polymer matrix around the pre-form. Methods for making pre-forms involve depositing chopped reinforcing fibers on to a porous form, consolidating the fibers by heating, and cooling to form a mat structure. For adequate consolidation and pre-form strength, a resin binder is usually incorporated before the heating step so that when heated, the resin flows across the fibers and acts as a binder. When the resin binder is a thermoset, a thermoset resin is incorporated before the heating step so that when heated, the thermoset resin flows across the fibers and acts as a thermoset binder when cured. The incorporation of a resin binder in order to economically produce adequate pre-forms continues to be a significant problem in the art.
One method of resin binder incorporation is known as the “wet laid” process involving the formation of an aqueous mixture of chopped glass fibers and resin binder, usually under agitation in a mixing tank. The resulting mixture may then be poured onto a porous mold or screen where suction is applied to remove liquid content. Another wet laid process involves depositing chopped fibers on a mold, spraying the fibers with an aqueous composition containing the resin binder. Alternatively, the chopped fiber can be sprayed with the aqueous composition while they are being chopped and deposited on the mold. The final step in all these processes is to heat the fibers on the mold causing the resin binder to flow and set. Unfortunately, due to the nature of glass fibers, it is difficult to obtain a uniform dispersion of fibers in a wet laid process. Poor fiber dispersion leads to undesirable qualities in a resultant pre-form such as poor structural strength. Additionally, a wet laid process often involves the use of volatile organic compounds (VOCs) and other organic solvents thereby triggering environmental safety concerns regarding their use.
In contrast to wet laid processes are “dry processes” where a resin binder is dry mixed with chopped fibers, heated to melt and cure the resin, and cooled to form a pre-form. Alternatively, a resin binder may be sprayed in molten form on to chopped fibers and cooled to form a pre-form. Unfortunately, an application of large quantities of resin binder is required in order to produce an acceptable pre-form. As a result, there is an undesirable generation of excess molten resin which may foul equipment and require extensive cleanup operations. Furthermore, when combined with a polymer matrix to form a reinforced composite, large quantities of resin binder present in a pre-form exacerbates any incompatibility between the resin binder and polymer matrix. Accordingly, there may be an increased likelihood of defects in the reinforced composite such as: 1) blistering, the result of undesirable chemical reactions between the resin binder and polymer matrix; 2) reduced bond strength between the pre-form and polymer matrix, also the result of undesirable chemical reactions; and 3) bleeding, the result of resin binder diffusing through the polymer matrix.
One solution to the problems caused by the aforementioned wet and dry processes is the production of string binders as disclosed in U.S. patent application Ser. No. 09/280,808, filed Mar. 30, 1999, entitled “String Binders” and U.S. patent application Ser. No. 09/593,550 filed Jun. 14, 2000, entitled “String Binders and Method for Making Same.” These references generally disclose a string binder made by incorporating molten thermoset resin on a strand composed of gathered fibers and cooling. These resin incorporated strands may then be chopped, directed to a mold, heated to cause the resin to flow, and cured into a thermoset resin binder. When cooled, a pre-form with a thermoset resin binder is formed.
Despite the advantages string binders provide in eliminating many of the cumbersome steps of conventional processes, the current methods for producing string binders require the handling of molten thermoset resins. It would be desirable to avoid the expense and difficulty of providing thermoset resins in molten form. The present invention satisfies such by producing a string binder with an aqueous chemical treatment.